/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* deleteDuplicates(ListNode* head) {
        ListNode* dummyHead = new ListNode(0);
        dummyHead->next = head; // 虚拟头节点
        ListNode* pre = dummyHead, *cur = head;
        while(cur != nullptr && cur->next != nullptr){
            ListNode* next = cur->next;  
            if(cur->val == next->val){
                pre->next = next; // 删除一个节点
            }
            else // 不用删除，更新节点
                pre = cur;
            cur = next;   
        }
        return dummyHead->next;
    }
};